1. Field of the Invention
This invention relates generally to fabrication of hermetic components containing lengths of optical fibers and, more particularly, to a method for continuously fabricating hermetic fiber optic-to-metal components and to the resultant hermetic fiber optic-to-metal components themselves.
2. Description of the Related Art
Fiber optics are used in a wide variety of applications ranging from telecommunications to medical technology and energetic components. Because of their unique structure, optical fibers are capable of highly accurate transmission of light, which is relatively unaffected by interference, diffusion, and other signal dehancing phenomena. In order for optical fibers to function at their optimum potential, however, they must be structurally intact and free of scratches, cracks, or leaks.
Optical fibers consist of a core material that is surrounded by a cladding. The difference between the indexes of refraction of the core and cladding materials (which, in some cases, are simply different types of fused silica glass) allows the optical fiber to function. Most commercially available optical fibers, in addition, have an external "buffer". The buffer is a thin coating (usually composed of a plastic, other polymer, or metal) which is applied to the fiber in order to protect it from being scratched during handling and to limit the amount of water than can come into contact with the fiber. Scratching or contact with water can deleteriously affect both the optical properties and the strength of the glass fiber. In addition to shielding the fiber's surface, the buffer also operates to help maintain the high tensile strength and the bending capability of the glass optical fibers.
A number of fiber optic applications require that one terminus of the fiber be located in an environment isolated from the other terminus. This implies the use of a connector or coupling device which serves as the point of communication between the distinct environments. Oftentimes, it is necessary or desirable for the point of communication between the environments to be completely sealed except for the presence of the optical fiber. Herein arises the need for a satisfactory method to hermetically seal optical fibers within metal fittings.
Fabrication of hermetic fiber optic-to-metal components has until recently been impractical due to a number of factors. Principal among these is the large thermal expansion mismatch between the very low coefficient of expansion of the optical fibers (most commonly made of fused silica glass) and the high coefficient of expansion of the metal shell to which the optical fibers are attached. This difference can cause severe stressing of the fiber optic components, especially where fabrication methods use application of heat, which, in turn, can cause undesirable cracks and leaks in the optical fibers.
Another problem with existing sealing methods concerns the use of polymer-buffered optical fibers. Methods for hermetically sealing fiber optic components typically involve the use of heat in the formation of the seal between the fiber and other elements of the fiber optic component. Heat deterioration of polymer buffer coatings, however, is one of the principal problems associated with creating seals involving optical fibers. Such deterioration, caused by the buffer burning off at the high temperatures required for most sealing processes, can result in the optical fibers having low tensile strengths and low bending strengths relative to undamaged fibers. Typically, the polymer buffer coating on an optical fiber will begin to burn off at about 220.degree. C. and will be over 90 percent removed at about 450.degree. C. These temperatures may be significantly below the temperature needed to form a high-quality seal.
Metal buffered optical fibers have been developed to overcome many of the problems associated with using polymer buffered fibers. Until now, however, attempts to fabricate hermetic fiber optic components using metal-coated optical fibers have been successful only with non-continuous "batch" heating operations. Formation of satisfactory fiber optic-to-metal seals could only be accomplished by using a batch furnace or flame heating. Due to the nature of these batch operations, throughput of processed parts is limited, and both processing time and the cost of batch-processed parts are increased relative to continuously-processed parts. For these reasons, existing batch techniques for fabrication of hermetically sealed fiber optic-to-metal components are commercially undesirable.
Disclosed here is a new method for fabricating hermetic fiber optic-to-metal components using continuous heat processing, such as with a belt furnace, and metal-coated fibers. The method of the present invention greatly enhances the commercial feasibility of fabricating hermetic fiber optic components, and the hermetic fiber optic components generated using the invention method are of notably high quality. In particular, they have been shown to exhibit significantly less stress damage and crack formation than hermetic fiber optic components made using other techniques.